In a conventional electrical double layer capacitor, polarizable positive and negative electrodes each consisting mainly of active charcoal or the like formed as an electrode layer on a collector are arranged facing each other with a separator sandwiched between the two to form the capacitor element. This capacitor element is contained together with an electrolyte solution in a metal case in a container, and insulated with a gasket between the metal case and sealing plate. Alternatively, the capacitor element may be formed by layering a separator between sheet-shaped polarizable electrodes consisting of a pair of positive and negative electrode sheets, and coiling the stacked sheets. This element is then impregnated with an electrolyte solution and contained in a metal case in a container, and the opening of the metal case is sealed with a seal member so that the electrolyte solution does not evaporate.
Such electrical double layer capacitors can be obtained in large quantities by using for the polarizable electrodes a metal with a valve action such as aluminum in the form of a sintered body, etched foil or the like. In other words, it is common to form a capacitor element by coiling a positive electrode foil and negative electrode foil of aluminum or another valve metal with a separator between them.
In such an electrical double layer capacitor, the electrolyte solution is an organic solvent such as propylene carbonate with a quaternary onium salt of boron tetrafluoride or phosphorus hexafluoride dissolved therein as a solute.
However, the moisture contained in the electrolyte solution or the like of such an electrical double layer capacitor produces an alkaline component by electrolysis at the negative electrode during aging. This alkaline component dissolves the oxide coat on the surface of the aluminum foil of the collector for example, and since the potential of the negative electrode during discharge is thus nobler than the solution potential of Al, the following reaction occurs within the collector.Al→Al3++3e-  [C1]
The problem has been that this reaction is accelerated in particular when F— or the like is present as a hydrolysis product of the BF4−anion used in the electrolyte, and since anionic compounds are formed at the same time, these compounds are electrolyzed and accumulate on the positive electrode. Thus, when such an electrical double layer capacitor is stored at high temperatures, the electrical properties deteriorate, with decreased electrostatic capacity and increased internal resistance.
The electrostatic capacity of a capacitor is affected by the type of insulating film formed on the electrode surface, the thickness of the electrode double layer and the surface area of the electrode, with electrostatic capacity being inversely proportional to the thickness of the electrode double layer. When the internal resistance of the capacitor is high, moreover, there is the possibility of a so-called IR drop, in which the voltage declines dramatically at the initial stage of discharge as the current density rises.
To solve such problems, an electrical double layer capacitor has been proposed whereby a rise in internal resistance can be prevented even during high-temperature no-load storage by improvements in the electrolyte solution (Patent Document 1). Specifically, an electrolyte solution is used having a tetrafluoroborate salt of quaternary ammonium as the solute and a mixture of γ-butyrolactone and propylene carbonate as the solvent.
Patent Document 1: Japanese Patent Application Laid-open No. 2003-109861
However, even the electrical double layer capacitor with improved electrolyte solution described in Patent Document 1 above has not succeeded in satisfactorily addressing the problem of decreased electrostatic capacity and increased internal resistance during storage in a high-temperature environment.